A living body (biological body) investing method (biopsy) is known to diagnose morbi (diseases) occurred in organs. In the biopsy, while organs located inside a body cavity are imaged by an ultrasonic imaging apparatus, a piercing needle is pierced up to a morbus (disease) portion so as to pickup biological body tissue of a region of interest (ROI) inside the piercing needle. The picked biological body tissue is discriminated to judge a disease name. However, after the biological body tissue has been extracted, or removed outside the living body, this extracted biological body tissue is fixed, cut in a thinner size, and stained so as to be investigated in the biopsy. As a consequence, this biopsy owns a problem that several weeks are necessarily required to diagnose such an extracted biological body tissue, another problem that the extracted biological body tissue is changed from a living state within the living body, and another problem that a tree-dimensional image can be hardly acquired.
To solve the above-described problems, the following needle-shaped ultrasonic probe has been proposed. That is, while the ultrasonic wave converter is mounted on the piercing needle, this piercing needle is directly pierced into the region of interest so as to measure the tissue characteristic stage of the region of interest, or to image living body tissue located around the region of interest. As the conventional needle-shaped ultrasonic probe, for instance, there are "PROBE MADE BY THAT CONCAVE IS FORMED IN NEEDLE, AND ULTRASONIC WAVE CONVERTER IS PROVIDED ON WALL SURFACE OF CONCAVE" (Japanese Patent Publication No. Hei-4-78299: first prior art), and "PROBE MADE BY THAT INNER NEEDLE OF PIERCING NEEDLE IS REPLACEABLE WITH ULTRASONIC WAVE CONVERTER" (Japanese Patent Publication No. Hei-6-125: second prior art). In the ultrasonic probes of the first and second prior art, acoustic characteristic such an sound velocities and reflectivities of living body tissue located around regions of interest are measured by employing ultrasonic waves, or ultrasound.
As another example of such an ultrasonic probe for imaging biological body tissue located around this ultrasonic probe based upon a acoustic characteristic (sound velocity and reflectivity) of this biological body tissue, the following prior art is known. For instance, there are "PROBE FOR SCANNING ULTRASONIC WAVE CONVERTER, MADE BY THAT WHILE OPENING PORTION IS FORMED IN A PORTION OF OUTER NEEDLE, ULTRASONIC WAVE CONVERTER MOUNTED ON SIDE SURFACE OF INNER NEEDLE IS EXPOSED TO OPENING UNIT" (Japanese Patent Publication No. Hei-5-9097: third prior art), and also "PROBE FOR EXPOSING INNER NEEDLE ON WHICH ULTRASONIC WAVE CONVERTER IS MOUNTED FROM TIP PORTION OF OUTER NEEDLE" ("ULTRASONIC IMAGING" magazine, volume 15, pages 1-13, 1993): fourth prior art).
The third prior art and the fourth prior art correspond to the arrangements for acquiring the images of the plane perpendicular to the axis of the needle, or the plane involving the axis of the needle, a so-called "B-mode image". The higher the frequency of the ultrasonic wave employed in the ultrasonic imaging operation is increased, the shallower the penetration depth by the ultrasonic wave becomes due to absorptions by the living body tissue. As a result, since the visual field is narrowed, the frequencies of the ultrasonic waves used in the imaging method capable of acquiring the B-mode image are selected to be lower than, or equal to approximately 100 MHz. To avoid such a problem that the penetration depth of the ultrasonic wave becomes shallow and thus the visual field is narrowed, which is caused by the condition that the ultrasonic wave converter with high resolution, operable in higher frequencies than, or equal to 100 MHz, another conventional method (Japanese Patent Application Laid-open No. Hei-8-154936; fifth prior art) has been proposed. That is, this imaging method acquires the image of the cylindrical plane (curved plane) around the needle, a so-called "cylindrical type C-mode" image.
In the ultrasonic probes of the fourth prior art and the fifth prior art, the ultrasonic wave converters equipped with the acoustic lenses are provided inside the needles in order to improve the lateral resolution. The ultrasonic waves which are produced from the piezoelectric transducer elements energized by the transmitted ultrasonic wave voltage are propagated within the acoustic lens materials to be converged. Then, the converged ultrasonic waves are reflected at the positions in the vicinity of the focal points of the acoustic lenses, so that the reflection signals are produced. The reflection signals are propagated through the opposite path to the piezoelectric transducer element to be converted into the voltages.
In general, when such an arrangement for improving lateral resolution with employment of an acoustic lens is used, a transmitted ultrasonic wave is a pulse wave so as to secure resolution along a depth direction (namely, radial direction while setting axis of needle as a center). In the case that such a pulsatory ultrasonic wave is transmitted, a distribution occurs in delay time defined until a reflection signal reaches a piezoelectric transducer element, depending upon a position of a reflector for reflecting a transmitted ultrasonic wave signal. In other words, the delay time may give positional information along the depth direction.
In the fourth prior art, while using such a fact that the delay time may give the positional information along the depth direction, the image along the depth direction is acquired. Also, in the fifth prior art, the imaging plane is set by time-gating the received ultrasonic waves on the wave reception side. In other words, while the signals having certain constant delay time are detected, the cylindrical plane is imaged, and this cylindrical plane is separated from the axis of the needle by a constant distance. In any cases, while the delay time and the time approximated to this delay time are time-gated so as to selectively detect the reflection signal reflected from the interiors of the living body tissue. The time gating operation given to the received ultrasonic wave signals may separate the transmitted ultrasonic wave signal from the received ultrasonic wave signal in the temporal manner, and therefore may avoid entering of the transmitted ultrasonic waves into the wave receiving device.
The method for transmitting the ultrasonic wave by using the burst wave is known in ultrasonic micro-scopes. In the case that the ultrasonic wave converter cannot be sufficiently energized by the pulse wave, since this ultrasonic wave converter is energized by using such a burst wave, S/N can be improved. Also, another method is known. That is, the multiple reflections within the acoustic lens material will interfere with the reflection signals by using the burst signal so as to improve the depth resolution.
The duration time of the burst wave used in the method for improving either S/N or the depth resolution is made shorter than the delay time defined between the ultrasonic wave transmission and he arrivals of the reflection signals (in most case, sufficiently shorter than delay time). Similar to the method with employment of the pulse wave, the time gate is provided on the reception side of the burst ultrasonic wave so as to separate the transmitted ultrasonic wave from the received ultrasonic wave.
To discriminate tissue from a characteristic state of biological body time, lateral resolution with a cell level is required. As is well known in this field, when a frequency of an ultrasonic wave is increased, a wavelength of the ultrasonic wave is shortened, so that lateral resolution may be increased. However, absorptions of ultrasonic waves by the living body tissue are also increased. Since the absorptions of the ultrasonic waves are increased, the strengths of the reflection signals are considerably decreased. This may cause such a problem that S/N would be lowered.
FIG. 21 represents attenuations of amplitudes, which are caused by absorptions by kidney and liver. Apparently, the absorptions are considerably emphasized in a frequency range higher than, or equal to 100 MHz.
It should be understood that the graphic representation shown in FIG. 21 is made by considering FIG. 4.10 (see page 176) in section 4 of publication entitled "Physical principles of medical ultrasonics".
The structures of the needle-shaped ultrasonic probes according to the first prior art and the second prior art own such a problem that since only one measuring point is obtained, sufficiently much information required for the diagnoses cannot be obtained. Also, in the arrangements for acquiring the B-mode image according to the third prior art and the fourth prior art, since the absorptions by the high frequency ultrasonic waves higher than, or equal to 100 MHz are increased, it is practically difficult to employ such high frequency ultrasonic waves higher than, or equal to 100 MHz. There is a problem that the resolution at the cell level could not be realized.
In the method for imaging the cylindrical plane around the needle by the C-mode image explained in the fifth prior art, it is possible to use such ultrasonic waves having the frequencies selected between 100 MHz and 200 MHz, so that the resultant resolution may be increased up to approximately 10 .mu.m. The resolution with on the order of 10 .mu.m is substantially equal to a dimension of a cell. In order to observe a characteristic state of biological higher resolution, it is desirable to use ultrasonic waves having frequencies nearly equal to 400 MHz. When the ultrasonic waves having the higher frequencies, i.e., on the order of 400 MHz are employed, since absorptions of these ultrasonic waves by the biological body tissue are increased, the strengths of the transmission signals need be largely increased at degrees higher than the degrees capable of compensating for the absorptions of the ultrasonic waves, and the S/N must be maintained.